This invention relates to the design of optical coatings for controlling the manner in which light of particular wavelengths is transmitted by an optical filter.
The phenomenon of optical interference, which causes modifications in the transmitted and reflected intensities of light, occurs when two or more beams of light are superposed. The brilliant colors, for example, which may be seen when light is reflected from a soap bubble or from a thin layer of oil floating on water are produced by interference effects between two trains of light waves. The light waves are reflected at opposite surfaces of the thin film of soap solution or oil. The principle of superposition states that the resultant amplitude is the sum of the amplitudes of the individual beams.
One important practical application for interference effects in thin films involves the production of coated optical surfaces. If a film of a transparent substance is deposited on glass, for example, with a refractive index which is properly specified relative to the refractive index of the glass and with a thickness which is one quarter of a particular wavelength of light in the film, the reflection of that wavelength of light from the glass surface can be almost completely suppressed. The light which would otherwise be reflected is not absorbed by a nonreflecting film; rather, the energy in the incident light is redistributed so that a decrease in reflection is accompanied by a concomitant increase in the intensity of the light which is transmitted.
Considerable improvements have been achieved in the antireflective performance of such films by using a composite film having two or more superimposed layers. In theory, it is possible with this approach to design a wide range of multiple-layer interference coatings for obtaining a great variety of transmission and reflection spectrums. This has led to the development of a large number of new optical devices making use of complex spectral filter structures. Antireflection coatings, laser dielectric mirrors, television camera edge filters, optical bandpass filters, and band-rejection filters are some examples of useful devices which employ thin-film interference coatings.
One type of bandpass filter is the cavity filter, which is a Fabry-Perot interferometer made with thin films. The cavity in the filter is a spacer layer which has an optical thickness equal to one half (or an integral multiple of one half) the wavelength of the light to be transmitted by the filter. The spacer layer is sandwiched between two reflecting layers which are made up of quarter-wave stacks of alternating high and low index layers, each layer having an optical thickness of one fourth the passband wavelength. Such a filter transmits at the wavelength for which the cavity thickness is a halfwave. Other wavelengths are reflected, thereby producing a bandpass filter.
The spacer layer essentially induces transmission through the two reflecting layers, which are tuned to reflect highly at the passband wavelength. This performance may be explained by the concept of absentee layers. Any halfwave layer (or a higher multiple of a halfwave layer) behaves in a multilayer stack as though it were absent. Therefore, if the spacer layer were removed in a cavity filter, the adjacent quarterwave layers on either side would effectively form another halfwave layer. Since this effective layer would also be a halfwave layer, this layer may also be denoted absentee and may be considered removed. This analysis can be continued by pairing quarterwave layers on either side of the cavity layer until only the substrate is left. Consequently, the transmittance of the cavity filter at the passband wavelength is theoretically the same as a bare transparent substrate.
The reflectance curve for an actual cavity filter, however, is considerably less ideal than the absentee layer theory implies. One reason for this is that the standing wave E field within the filter exhibits some high peaks, particularly for a narrowband filter, at the interfaces between the spacer layer and the adjacent reflecting layers. If there is high absorption or damage sites at these interfaces, a considerable amount of the passband energy can be absorbed in the filter rather than being transmitted. This effect is exacerbated by the need for halfwave layers in the cavity layer, since in the deposition process the thicker halfwave layers tend to develop more irregularities at their interfaces than do thinner layers. These irregularities further contribute to the absorption of the passband wavelengths in prior art cavity filters and make high transmission particularly difficult to attain in a narrow bandwidth filter.